A torque converter includes a torus having three types of vane wheels (an impeller, a turbine, and a stator) and serves to transmit torque by fluid in the torus. The impeller forms a fluid chamber filled with operating oil with the front cover. The impeller is mainly made of an annular impeller shell, a plurality of impeller blades fixed to an inner surface of the impeller shell, and an annular impeller core fixed to the inner ends of the impeller blades. The turbine is disposed in the interior of the fluid chamber opposing the impeller in the axial direction. The turbine is mainly made of an annular turbine shell, a plurality of turbine blades fixed to a surface of the turbine shell facing the impeller, and an annular turbine core fixed to the inner ends of the turbine blades. A radially inner portion of the turbine shell is fixed to a flange of a turbine hub by a plurality of rivets. The turbine hub is non-rotatably coupled to a main drive shaft of the transmission. The stator regulates the flow of the operating oil that is returned from the turbine to the impeller. The stator is disposed between radially inner portions of the impeller and the turbine. The stator is mainly made of an annular stator shell, a plurality of stator blades formed on an outer circumference of the stator shell, and an annular stator core formed on tips of the stator blades. The stator is supported by a stator shaft via a one-way clutch.
In general, a torque converter can smoothly accelerate and decelerate because power is transmitted by fluid. However, a loss of energy can be caused by fluid slip resulting in poor fuel economy. Therefore, a torque converter that is mounted with a lock-up device to connect mechanically an input-side front cover and an output-side turbine exists among prior art torque converters. The lock-up device is disposed in a space between the front cover and the turbine. The lock-up device is mainly formed of a disc-shaped piston, a driven plate, and torsion springs. The disc-shaped piston can be frictionally engaged with the front cover. The driven plate is mounted to a back face side of the turbine. The torsion springs elastically connect the piston and the driven plate in a rotating direction. An annular frictional member is adhered to the piston so that it faces a flat frictional face of the front cover (refer to Unexamined Patent Publication 2003-56669, for example).